Bottom Line:
Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions.Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion.The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

ABSTRACTAlthough glucose uniquely stimulates proinsulin biosynthesis in β cells, surprisingly little is known of the underlying mechanism(s). Here, we demonstrate that glucose activates the unfolded protein response transducer inositol-requiring enzyme 1 alpha (IRE1α) to initiate X-box-binding protein 1 (Xbp1) mRNA splicing in adult primary β cells. Using mRNA sequencing (mRNA-Seq), we show that unconventional Xbp1 mRNA splicing is required to increase and decrease the expression of several hundred mRNAs encoding functions that expand the protein secretory capacity for increased insulin production and protect from oxidative damage, respectively. At 2 wk after tamoxifen-mediated Ire1α deletion, mice develop hyperglycemia and hypoinsulinemia, due to defective β cell function that was exacerbated upon feeding and glucose stimulation. Although previous reports suggest IRE1α degrades insulin mRNAs, Ire1α deletion did not alter insulin mRNA expression either in the presence or absence of glucose stimulation. Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions. In contrast, Ire1α deletion in β cells increased the expression of over 300 mRNAs encoding functions that cause inflammation and oxidative stress, yet only a few of these accumulated during high glucose. Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion. The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

Mentions:
One of the most remarkable features of the pancreatic beta cells—those that produce and secrete insulin to regulate glucose homeostasis—is their capacity to increase the synthesis of proinsulin (the insulin precursor) up to 10-fold after glucose stimulation. This dramatic increase in the synthesis of proinsulin is a challenge to the proximal secretory pathway and triggers an adaptive stress response, the unfolded protein response, which is coordinated by the IRE1α enzyme and the X-box-binding protein 1 (XBP1) transcription factor. Deletion of IRE1α specifically from the pancreatic beta cells in adult mice resulted in overt diabetic phenotypes such as high blood glucose. mRNA analysis revealed several hundred genes whose expression was coordinately regulated by glucose and IRE1α and whose functions are important for the beta cell secretory pathway. Furthermore, IRE1α also regulates the expression of mRNAs involved in the production of reactive oxygen species (ROS), and we could show that, in fact, oxidative stress is a primary mechanism that causes beta cell failure upon collapse of the secretory pathway. Finally, in experiments with murine and human islets (the regions of the pancreas where secretory beta cells are located), we observed that while IRE1α does not regulate the expression of the gene encoding insulin, it determines final insulin levels by controlling translation of proinsulin mRNA.

Mentions:
One of the most remarkable features of the pancreatic beta cells—those that produce and secrete insulin to regulate glucose homeostasis—is their capacity to increase the synthesis of proinsulin (the insulin precursor) up to 10-fold after glucose stimulation. This dramatic increase in the synthesis of proinsulin is a challenge to the proximal secretory pathway and triggers an adaptive stress response, the unfolded protein response, which is coordinated by the IRE1α enzyme and the X-box-binding protein 1 (XBP1) transcription factor. Deletion of IRE1α specifically from the pancreatic beta cells in adult mice resulted in overt diabetic phenotypes such as high blood glucose. mRNA analysis revealed several hundred genes whose expression was coordinately regulated by glucose and IRE1α and whose functions are important for the beta cell secretory pathway. Furthermore, IRE1α also regulates the expression of mRNAs involved in the production of reactive oxygen species (ROS), and we could show that, in fact, oxidative stress is a primary mechanism that causes beta cell failure upon collapse of the secretory pathway. Finally, in experiments with murine and human islets (the regions of the pancreas where secretory beta cells are located), we observed that while IRE1α does not regulate the expression of the gene encoding insulin, it determines final insulin levels by controlling translation of proinsulin mRNA.

Bottom Line:
Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions.Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion.The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

ABSTRACTAlthough glucose uniquely stimulates proinsulin biosynthesis in β cells, surprisingly little is known of the underlying mechanism(s). Here, we demonstrate that glucose activates the unfolded protein response transducer inositol-requiring enzyme 1 alpha (IRE1α) to initiate X-box-binding protein 1 (Xbp1) mRNA splicing in adult primary β cells. Using mRNA sequencing (mRNA-Seq), we show that unconventional Xbp1 mRNA splicing is required to increase and decrease the expression of several hundred mRNAs encoding functions that expand the protein secretory capacity for increased insulin production and protect from oxidative damage, respectively. At 2 wk after tamoxifen-mediated Ire1α deletion, mice develop hyperglycemia and hypoinsulinemia, due to defective β cell function that was exacerbated upon feeding and glucose stimulation. Although previous reports suggest IRE1α degrades insulin mRNAs, Ire1α deletion did not alter insulin mRNA expression either in the presence or absence of glucose stimulation. Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions. In contrast, Ire1α deletion in β cells increased the expression of over 300 mRNAs encoding functions that cause inflammation and oxidative stress, yet only a few of these accumulated during high glucose. Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion. The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.